asymptotic expansion
From Glossary of Meteorology
A series of the form c0 + (c1/x) + (c2/x2) + . . . + (cn/xn) + . . . often used for formal solution of differential equations or for function approximation when 
is large.
Though such series are divergent, terms initially decrease in size to one of minimum value at which term the series can be truncated to obtain a useful approximation.
